Thermoplastic olefins (TPO's), which typically comprise polypropylene and an elastomer, have many desirable properties, e.g., lightweight, durability, low cost, etc., that make them an attractive material of construction for many interior and exterior automotive parts. There are two types of TPO's which essentially differ in their method of preparation. The first, which is commonly referred to as "compound grade TPO", is made by physically blending polypropylene with elastomer. The second, which is commonly referred to as "reactor grade TPO" and is more economical to produce than compound grade TPO, is made by first polymerizing propylene to polypropylene and then polymerizing elastomer components, such as ethylene and propylene, in the presence of the polypropylene.
In an attempt to reduce the costs of automotive parts manufactured from TPO's, high flexural modulus TPO's (flexural modulus above about 130,000 psi.) have been used in place of lower flexural modulus TPO's. In employing the use of high flexural modulus TPO over lower flexural modulus TPO's, a raw materials cost savings can be realized since the part may be made thinner than if made with a low flexural modulus TPO due to the increased relative flexural moduli of the TPO's.
Presently, all paintable high flexural modulus TPO's (flexural modulus above 130,000 psi.) are made by blending (i.e., compounding) polypropylene with elastomer. Accordingly, in an effort to further reduce the costs of automotive parts manufactured from high flexural modulus TPO's, it is an object of the present invention to provide reactor grade, high flexural modulus TPO's.
Many of the automotive parts manufactured from TPO's are painted to improve the aesthetic quality of the automotive part. For instance, automobile bumpers formed from TPO's are often painted to match the color of the rest of the automobile's exterior paint. Accordingly, it is another object of the present invention to provide reactor grade, high flexural modulus TPO's having acceptable paint adhesion.
As the use of painted TPO automotive parts has become more prevalent, it has been observed that painted TPO automotive parts are relatively susceptible to damage known as compressive-shear induced delamination (i.e., gouging) as a result of compressive shear loads acting on the part. An example of such a compressive shear load is a shopping cart impacting an automobile bumper. Accordingly, it is a further object of the present invention to provide reactor grade, high flexural modulus TPO's having acceptable paint adhesion and gouge resistant for use in the manufacture of automotive parts.